High intensity, broad bandwidth light sources are useful for interferometric measurements such as optical coherence domain reflectometry (OCDR), optical coherence tomography (OCT), and self-interference interferometry (SII). High intensity sources are desirable to achieve a large number of constructively interfering photons received back from a weakly reflecting sample that can be observed on a detector. A continuous, smooth, wide spectrum of optical frequencies is desirable to achieve high spatial resolution in the direction of optical propagation.
OCDR is an interferometric imaging method that determines the scattering profile of a sample along the beam by detecting light reflected from a sample combined with a reference beam. Each scattering profile in the depth direction (z) is called an axial scan, or A-scan. OCT is an extension of OCDR in which cross-sectional images (B-scans), and by extension 3D volumes, are built up from many A-scans, with the OCT beam moved to a set of transverse (x and y) locations on the sample. Modern OCT systems typically collect spectrally resolved data because this allows simultaneous measurement of a range of depths at high depth resolution, without a signal to noise penalty. This method of collecting spectrally resolved data and translating it to a depth resolved measurement via a Fourier transform across the spectral dimension is referred to as “frequency domain” or “Fourier domain” OCT (FD-OCT).
Traditional sources for FD-OCT can be described as producing a broad bandwidth light simultaneously as with superluminescent diodes (SLD) or femtosecond Titanium Sapphire lasers; or as producing a broad bandwidth by sequentially tuning through a range of narrow bandwidths which, when considered together, constitute a broad time integrated bandwidth. The latter type of source may be called a swept source. Examples of swept-sources include external cavity tunable lasers (ECTL), vertical cavity surface emitting lasers (VCSEL), and sampled grating distributed Bragg reflector lasers (SG-DBR) among others. Simultaneous broad-bandwidth sources are typically detected with a spectrally dispersing element and a linear array of photodiodes in what is called as spectral domain OCT (SD-OCT). Swept sources are typically detected over time using one or two single element photo-detectors, therefore encoding the spectral information in the time dimension, in what is referred to as swept-source OCT (SS-OCT). Exposure times for SD-OCT systems in the human eye are limited by phase washout (i.e., cancellation of signal during a measurement period due to axial/lateral motion) to about 100 μs. Exposure times for SS-OCT systems are limited to about 1 ms. A hybrid swept source, spectral domain (SS/SD-OCT) arrangement has been described which allowed longer exposure times than a standard SD-OCT system without phase washout (see for example, Yun, S. H., Tearney, G., de Boer, J., & Bouma, B. (2004). Pulsed-source and swept-source spectral-domain optical coherence tomography with reduced motion artifacts. Optics Express, 12(23), 5614-5624).
Both SD-OCT and SS-OCT systems have found use in commercial applications, a notable example being the field of ophthalmology. The high brightness simultaneous or swept broad bandwidth sources used in today's commercial ophthalmic OCT systems account for a significant fraction of the total system cost. SS-OCT is a rapidly developing area of OCT; however the cost remains very high, due to which widespread commercial acceptance has not been met yet.
Parallel OCT systems simultaneously illuminate a set of A-scans, whereas traditional point scanning OCT systems illuminate a single lateral point at a time. Field illumination OCT is a subset of parallel OCT where the illumination is contiguous between multiple A-scans (as a line, partial field, or full field) as opposed to spatially separated individual A-scans. Field illumination OCT offers potential benefits in terms of cost relative to point scanning OCT, partly owing to simplifications in fast beam scanning Parallel OCT systems require similar exposure energy per unit area as traditional point-scanning OCT systems, to achieve similar shot noise limited sensitivity. The constraints on exposure energy and exposure time result in greater source power requirements for parallel systems compared to traditional point-scanning OCT to achieve similar signal-to-noise ratio (SNR) in a single measurement. Some highly parallel SD-OCT systems use two-dimensional (2D) array sensors to measure many points of spectra simultaneously. 2D sensors are commonly available as consumer electronics such as cell phone and security cameras, and can therefore often be found for less than the cost of linear arrays typically found in point scanning SD-OCT systems. The frame rates of these consumer devices are currently typically less than 200 Hz. The short exposure times of SD-OCT, and the slow frame rates of low cost 2D arrays imply a duty cycle less than 2%. This low duty cycle means that an attempt to construct composite scans from serially acquired scans illuminated with a continuous wave (CW) source will have low efficiency (i.e., over time, more than 98% of the power from the source will need to be blocked). Existing superluminescent diodes are moderately expensive and insufficient in terms of power, and existing swept sources are too expensive to power low cost OCT devices that take advantage of the cost benefits of field illumination OCT.
Semiconductor diode lasers can easily achieve single transverse mode power levels on an order of magnitude larger than superluminescent diodes (SLD). When a semiconductor diode material begins to lase, the spectral bandwidth narrows, usually to one or a few very narrow peaks called longitudinal modes which are constrained by the resonance of the laser cavity. Bandwidth as used herein refers to the width of the envelope of the multiple longitudinal modes. Typically, consumer devices such as compact disc readers, laser printers, and rangefinders need the high intensity and spatial coherence provided by a laser, but do not require, or find it disadvantageous to employ a broad bandwidth such as provided by an SLD. Although SLD and semiconductor diode lasers largely share the same materials, manufacturing, and packaging techniques, semiconductor diode lasers are often manufactured at low unit costs because of the extremely high volumes utilized in consumer electronics.
Methods of tuning and shaping the spectral output of diode lasers have been developed and employed for various applications including optical coherence tomography and related interference techniques. The temperature dependence of the semiconductor bandgap was one of the first, and most common, methods demonstrated to tune the output of a diode laser, but is usually associated with a response that is too slow and coarse for OCT. A closed loop system has been described in the art where a current controlled thermocouple adjusts the case temperature of the laser package (see for example, Bartl, J., Fira, R., and Jacko, V. (2002). Tuning of the laser diode. Measurement Science Review volume 2 section 3, hereby incorporated by reference). Rapidly tunable intracavity filters, which precisely restrict the longitudinal mode of the laser, are standard in swept source OCT (see for example, U.S. Pat. No. 5,949,801 hereby incorporated by reference), where it is desirable to smoothly sweep a narrow laser line across a broad bandwidth. The complexity associated with this method results in a high system cost.
The total spectrum from multi-longitudinal mode lasers, which simultaneously produce many closely spaced, narrow bandwidth modes, is relatively broad; however the comb structure on the spectrum causes severe sidelobe artifacts. Such multi-longitudinal mode laser spectra can be blurred over time by forcing the comb spectrum to shift slightly during the measurement period so that the comb peaks move to fill in the spaces in between the peaks. Non-equilibrium thermal effects and carrier density effects can create small changes in refractive index of the laser cavity. These changes in refractive index effectively change the optical length of the laser cavity such that the modes of the cavity shift slightly to blur the comb structure. Wei-Kuo Chen demonstrated the smoothing of a comb spectrum for a depth ranging application by driving a multimode laser with 100 picosecond long pulses which act primarily by changing the optical length of the cavity (by carrier density effects) to shift its resonances (see for example, Wei-Kuo Chen and Pao-Lo Liu, “Short-coherence-length and high-coupling-efficiency pulsed diode laser for fiber-optic sensors,” Opt. Lett. 13, 628-630 (1988), hereby incorporated by reference). Such short pulses are difficult to achieve because of the impedance of the drive electronics and the packaging of the diode laser itself.
An interferometric imaging system closely related to OCT was demonstrated using a multimode diode laser and applying a sinusoidal 100 Hz modulation between lasing threshold and approximately max sustainable CW current (see for example, Balboa, I., Ford, H. D., and Tatam, R. P. (2006). Low-coherence optical fibre speckle interferometry. Measurement Science and Technology 17, 605, hereby incorporated by reference). At this much slower modulation frequency, thermal effects are believed to dominate over carrier density effects. The comb structure is blurred to become more ideally Gaussian (therefore suppressing sidelobes), and is broadened by a very modest fraction of 1.2 from 3.2 nm to 4.0 nm ultimately delivering a relatively coarse axial resolution of 165 μm.
In light of the limitations of the current state of the art, there is a need for low cost sources and detector arrangements for use in interferometric imaging systems.